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Z2 workshop report

Drifters in deep pelagic environments - A workshop over the hill from Nice, France

During Leg I and II of the GO Sars Expedition in July 2004 a plethora of new information about the distribution and abundance of particulate matter and zooplankton along the mid-Atlantic ridge (MAR) was obtained from vertical surveys with optical technologies (ROV and UVP) and traditional plankton nets (multi-net and 1-m net).  Members of the Z2 group convened in early April 2006 at the historic Station Zoologique in southern France (also known as l'Observatoire Océanologique de Villefranche-sur-mer) to continue analyses of these unique data.

l'Observatoire Océanologique de Villefranche-sur-Mer is a dynamic research facility nestled in historic (dating back to 1792) buildings situated along the western shoreline of a picturesque bay.  This setting provided a pleasant milieu in which to analyze zooplankton data from the mid-Atlantic.  Gaby Gorsky was the quintessential host and generously shared his office and laboratory (Fig. 1). The weather was crisp but sunny nearly every day. 

We easily adapted to French customs by periodic infusions of high-octane espresso and early morning consumption of fresh-baked croissants, which kept our minds alert and our stomachs sated. In the above overview of Villefranche Bay, l'Observatoire Océanologique (also known as Station Zoologique) is located just above the harbor.

Participants included Aino Hosia (University of Bergen) and Marsh Youngbluth (Harbor Branch Oceanographic Institution) plus Marc Picheral, Lars Stemmann, and Gaby Gorsky (Laboratoire d' Oceanographie de Villefranche-sur-mer).  Tom Sørnes (Hydro in Bergen) and Francesc Pagès (Institut de Ciences del Mar, Barcelona) were unable to attend the workshop but nevertheless provided pre- and post-workshop contributions to ongoing analyses.

Activities focused mainly on number crunching with statistical software packages (MATLAB and PRIMER) using hierarchical clustering and anosim methods.  These efforts more rigorously defined vertical distribution patterns of fauna and their associations with water column features and water mass characters.  Video records were also reviewed to verify identifications of pelagic fauna, mostly the small mesozoplankton.

Several major conclusions were confirmed:
1. the biodiversity of gelatinous zooplankton is low in the open ocean near the MAR;
2. the most frequently observed gelatinous fauna in order of overall abundance included medusae, ctenophores, siphonophores, appendicularians, and tunicates. 
3. the majority of these animals are relatively small (1-5 cm);
4. fewer species occurred at the northernmost stations;
5. different water masses encountered in the upper 100 m along the mid-Atlantic ridge harbored dissimilar assemblages of zooplankton. 
6. the largest biomass and highest abundance of gelatinous fauna consistently occurred in the mesopelagic zone from 500-800 m.

During the course of our visit the latest generation of a zooplankton imaging system, called ZOOSCAN, was demonstrated by Marc Picheral.  This device provides a rapid means of counting and sizing zooplankton collected with nets (see image).  Software packages tabulate and plot these data in a variety of ways.


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